Recording performance for magnetic disks is commonly determined by three basic characteristics--PW50, overwrite (OW), and noise. PW50 is the pulse width of the bits at half-maximum, expressed in either time or distance. A narrower PW50 allows for higher recording density, while a wide PW50 means that the bits are crowded together, resulting in adjoining bits interfering with one another. This interference is termed inter-symbol interference. Excessive inter-symbol interference limits the linear packing density of bits in a given track, hence reducing the packing density in a given area, and hence limiting the recording capacity of the magnetic media.
One means of reducing PW50 is to reduce the thickness of the magnetic layer of the media (the media being comprised of at least a substrate, a magnetic layer, an overlayer, and possibly additional layers). Another means of reducing PW50 is to increase hysteresis loop squareness ("S", including coercivity squareness "S*" and remanent coercivity squareness "S*.sub.rem "), and narrow the switching field distribution ("SFD"), as described by William and Comstock in "An Analytical Model of the Write Process in Digital Magnetic Recording," A.I.P. Conf. Proc. Mag. Materials 5, p. 738 (1971). Yet another means for reducing the PW50 is to increase the coercivity ("Hc") of the media.
Overwrite ("OW") is a measure of the ability of the media to accommodate overwriting of existing data. That is, OW is a measure of what remains of a first signal after a second signal (for example of a different frequency) has been written over it on the media. OW is low, or poor when a significant amount of the first signal remains. OW is generally affected by the coercivity, the squareness, and the SFD of the media. For future high density recording, higher Hc media will be preferred. However, gains in He are generally accompanied by losses in OW. Thus, there is a need in the art to improve the S* and the SFD to obtain improvements in OW.
Noise performance of a magnetic film is typically defined in terms of read jitter and write jitter. Read jitter is primarily determined by the amount of signal available from a bit, and the electronic noise in the channel. A thicker magnetic film will typically provide reduced read jitter. Unlike read jitter, write jitter is determined by the intrinsic noise of the film. Intrinsic media noise has been theoretically modeled by Zhu et al. in "Micromagnetic Studies of Thin Metallic Films", J. Appl. Phys., vol. 63, no. 8, p. 3248 (1988), which is incorporated by reference herein. Chen et al. describe the source of intrinsic media noise in "Physical Origin of Limits in the Performance of Thin-Film Longitudinal Recording Media," IEEE Trans. Mag., vol. 24, no. 6, p. 2700 (1988), which is also incorporated by reference herein. The primary source of intrinsic noise in thin film media is from interparticle exchange interaction. In general, a higher interparticle exchange results in higher S* and lower SFD due to the co-operative switching of magnetic grains. However, a high exchange interaction results in high noise.
The noise from interparticle exchange interaction can be reduced by isolating the individual particles (grains). This may be accomplished by physically spacing the grains apart from one another as described by Chen et al. in the aforementioned "Physical Origin of Limits in the Performance of Thin-Film Longitudinal Recording Media". The amount of separation need be only a few angstroms for there to be a significant reduction in interparticle exchange interaction.
There is another interparticle interaction, called magnetostatic interaction, which acts over a much greater distance between particles as compared to the exchange interaction. Reducing the magnetostatic interaction does reduce intrinsic media noise slightly. However, the effects of magnetostatic interaction actually improve hysteresis loop squareness and narrow the switching field distribution (but to a lesser extent than the exchange interaction), and hence improve PW50 and OW. Therefore, magnetostatic interaction is generally desirable and hence tolerated.
In order to obtain the best performance from the magnetic media, each of the above criteria--PW50, overwrite, and noise--must be optimized. This is a formidable task, as each of these performance criteria are inter-related. For example, obtaining a narrower PW50 by increasing the Hc will adversely affect overwrite, since increasing He degrades overwrite. A thinner media having a lower remanent magnetization-thickness product ("Mrt" where Mr is the remanent magnetization and t is thickness of the magnetic layer) yields a narrower PW50 and better OW, however the read jitter increases because the media signal is reduced. Increasing squareness of the hysteresis loop contributes to narrower PW50 and better OW, but may increase noise due to interparticle exchange coupling and magneto static interaction. Since it is a known goal to reduce or eliminate interparticle exchange coupling, the amount that PW50 may be narrowed and OW improved has heretofore been limited by the increase in tolerable noise level arising from the magnetostatic interaction of the media.
Therefore, an optimal thin film magnetic recording media for high density recording applications, i.e., that can support high bit densities, will require low noise without adversely sacrificing PW50 and OW. Recording density can then be increased since bit jitter is reduced. One type of magnetic media which has allowed optimizing certain of the above performance criteria is based on alloys of cobalt (Co) and platinum (Pt), due to the alloys' ability to provide high Hc and high magnetic moment.
The media noise of CoPt based alloys can be reduced by a number of different approaches, but as described in the following, these methods suffer from loss of hysteresis loop squareness (i.e., lower S* and higher SFD), increased PW50, decreased OW, and other disadvantages. One such approach teaches about deposition of the magnetic alloy by sputtering in a high argon pressure environment, as has been described by Chen et al. in the aforementioned "Physical Origin of Limits in the Performance of Thin-Film Longitudinal Recording Media". Basically, the application of high argon pressure results in isolated, exchange decoupled grains. Although the media noise is reduced, S* and OW decreased, and SFD increased resulting in an increase in PW50.
In another approach, in order to decrease the media noise, it is known to introduce oxygen into the magnetic film in a concentration of 5 to 30 atomic percent (at. %), as taught by Howard et al. in U.S. Pat. No. 5,066,552, (see also Howard et al.'s U.S. Pat. No. 5,062,938, which teaches oxidizing the magnetic grains after growth). Howard et al. teach the formation of a magnetic layer by vacuum sputtering in an argon atmosphere into which oxygen has been introduced. Oxygen is therefore introduced into the magnetic layer from the sputtering environment. However, as pointed out by Howard et al. in said patent, introducing oxygen decreases both Hc and S*.
There are a number of disadvantage to the approaches taught by Howard et at. ('938) First, the additional step of oxidizing a sputtered layer after depositing an impurity adds to the manufacturing complexity and cost. Second, Howard et al. teaches nothing about controlling the formation of the oxides. Third, Howard et al. teaches nothing about controlling the grain size and grain uniformity.
Yet another approach is to make granular films having grains of magnetic alloys containing SiO.sub.2. Details about these films have been described by C. L. Chien et al. in "Magnetic Granular Fe--SiO.sub.2 Solids", J. Appl. Phys., 61(B), p. 3311 (1987), and S. H. Liou et al. in "Granular Metal Films a recording Media", Appl. Phys. Lett., 52(8), p. 512 (1988). Essentially, these researchers were depositing Fe--SiO.sub.2 either by co-sputtering or by using composite targets and the magnetic films were deposited without underlayers. The values for Hc of around 1100 Oe and for squareness of around 0.6 were obtained. These values are unacceptably low for high density recording applications.
Similarly, addition of SiO.sub.2 has been exploited by Shimizu et al. as described in "CoPtCr Composite Magnetic Thin Films", IEEE Trans. Mag., vol. 28, no. 5, page 3102 (1992), and its companion patent applications: European Patent Application 0 531 035 Al, published Mar. 10, 1993, and Japanese Patent Application 5-73880, published Mar. 23, 1993. Specifically, lower media noise and higher in-plane coercivity were noted with an introduction of approximately 10% by volume (vol. %) SiO.sub.2. The S* of these films were generally around 0.6, as discussed in the aforementioned paper of Shimizu et al. Thus, although media including SiO.sub.2 showed lower media noise and higher Hc, the squareness obtained was again too low to meet the requirements for high density recording. It should also be noted that Shimizu et al. required approximately 17-18 at. % of Pt in the alloys. Such high percentage of Pt significantly increases the manufacturing cost of such media (although for media designed for use with magneto-resistive heads, e.g., having an Mrt of about 1.0 memu/cm.sup.2, a higher platinum content, such as 18%, may be required to maintain Hc). Furthermore, it should be noted that Shimizu et al. achieved a peak Hc of 1700 Oe, an unacceptable limit for future high density recording applications.
Another approach, as discussed in Japanese Patent Application 5-197944, published Aug. 6, 1993 (Murayama et al.) is use of SiO.sub.2 for increased Hc while sputtering in the presence of a broad range of N.sub.2, for example 0.1 to 10% on a NiP under layer. Lower media noise was obtained, but at the cost of decreasing S* as the percentage of SiO.sub.2 increased. Thus, lower media noise was obtained at the cost of increasing PW50 and OW. Additional teachings relating to SiO.sub.2 may be found in U.S. Pat. Nos. 4,837,094 to Kudo (teaching an amorphous alloy) and 4,769,282 to Tada et al. (teaching an alloy including rare earth elements). Importantly, all of the references to SiO.sub.2 teach alloying or admixing the SiO.sub.2 with the magnetic film constituents, as opposed to depositing SiO.sub.2 and the magnetic film constituents under conditions such that there is co-deposition but only minimal alloying of the SiO.sub.2 with the magnetic film material.
There are a number of disadvantages to the alloying or admixing of impurities, for example as taught by Shimizu et al. First, the addition of an impurity material (e.g., up to 30 vol. % SiO.sub.2 by Shimizu et al.) results in a decrease in Ms and hence a decrease in Mr. Therefore, the thickness of the magnetic layer must be increased to maintain sufficient Mrt. This is undesirable because an increase in film thickness is generally accompanied by an increase in space loss between the head and the media, which results in a larger PW50 and worse OW. Second, the sputtering process is made more complex and more costly by the requirement that additional materials be sputtered. Third, the alloyed or admixed impurity does nothing to increase grain isolation to thereby reduce exchange coupling induced noise.
Murdock et al. in "Noise Properties of Multilayered Co-Alloy Magnetic Recording Media", IEEE Trans. Mag., vol. 26, p. 2700-2705 (1990) teach deposition of multiple layers of magnetic material isolated from each other by layers of nonmagnetic material to reduce media noise. It is theorized that grain size and distribution may be relatively controlled several grains in thickness above an under layer. As a film grow thicker, the grains tend to vary in size and position. Thus, Murdock et al. teach controlling grain size and spacing by deposition of an under layer, forming a thin magnetic layer thereon several grains in thickness, forming on this magnetic layer another under layer, forming on that under layer another thin magnetic layer, and so on. Although media noise is reduced due to smaller isolated grains, the SFD is increased and the squareness is reduced due to a difficulty in matching the Hc of the individual layers.
Moreover, the grain size of the thinner magnetic layers may be reduced so much that the magnetic grains may become superparamagnetic, resulting in a dramatic decrease in Hc. Manufacturing of such multilayered films is also very difficult and requires additional process chambers over current equipment requirements. In addition, special attention is needed to design the manufacturing process to minimize oxidation of thinner magnetic layers. Thus, although the multi-layer approach does teach a method for reducing media noise, the squareness degrades and the process is difficult and expensive.
Currently, there are recognized limits on the ability to obtain high squareness and low media noise simultaneously. This is especially true with regard to isotropic media. This problem has lead to compromises in the values of the magnetic performance parameters, ease and cost of manufacturing, etc., for longitudinal recording media for high recording density. See, for example, Yogi et al., "Longitudinal Media for 1 Gb/in.sup.2 Areal density", IEEE Trans. Mag., vol. 26, page 2271 (1990). Therefore, there is at present a need in the art for a method of reducing media noise without compromising other media performance characteristics such as high coercivity, high squareness (high S* and lower SFD), high SNR, high overwrite, and low PW50. This is becoming crucial for high density applications as recording densities approach (or exceed) 10 Gb/in.sup.2.